1. Field of the Invention
The present invention relates to an electric motor-driven power steering apparatus.
2. Description of the Related Art
Conventionally, an electric motor-driven power steering apparatus in which improvements in fuel consumption can be achieved using an electric motor as power has been commercialized with respect to an oil hydraulic power steering apparatus using oil pressure as power.
For example, as shown in FIG. 3, a controller of such an electric motor-driven power steering apparatus comprises a bridge circuit constructed of four MOS field-effect transistors (hereinafter called an FET) Q1 to Q4, and a battery B is connected between the input terminals and a DC motor M is connected between the output terminals and by performing ON driving or PWM driving of the FETs forming opposite sides, driving is performed in the right direction or the left direction.
Also, in case that a fault (hereinafter called an ON fault) resulting in an ON state even in a non-driving state occurs in one of the FETs Q1 to Q4 constructing the bridge circuit (in FIG. 3, case that an ON fault occurs in Q3), even when the controller detects this ON fault and stops control, a closed circuit made of the DC motor M and parasitic diodes of the FETs Q3 and Q4 is constructed and when a driver steers, the DC motor M acts as a power generator and a power generation current (braking current) flows (shown by an arrow of a solid line in FIG. 3), so that there was a problem that steering force necessary for the driver to steer becomes larger than the case of merely stopping control (that is, manual steering force) and steering feeling is remarkably made worse.
In order to solve this problem, as shown in FIG. 4, a motor relay R1 is inserted between the output terminal of the bridge circuit and the DC motor M and in case that some abnormality including an ON fault occurs in the controller, by opening this motor relay R1, the construction of the closed circuit described above was prevented and the necessary steering force was prevented from becoming large.
Since parasitic diodes are present in the FETs Q1 to Q4 constructing this bridge circuit in its structure, in case of making connection by wrong polarity when the battery is connected between the input terminals of the bridge circuit (shown by a broken line of FIG. 3), there is a problem that a short-circuit current flows through the parasitic diodes (shown by an arrow of the broken line in FIG. 3) and the FETs Q1 to Q4 are destroyed, and in order to prevent this, a power source relay R2 of normal open was inserted between the input terminal and the battery as shown in FIG. 4.
Also, in order to solve this problem similarly, it is proposed that a two-contact relay should be inserted into each the input terminal of the bridge circuit.
In the controller as described above, it is necessary to insert the relay in any case. Additionally, in order to solve both of the problems described above, it is necessary to insert two relays. It is necessary to pass an electric motor current through this relay at the normal time and a current of several tens of amperes flows in the electric motor-driven power steering apparatus, so that it was necessary to use a large-size relay with a large current capacity and it resulted in an obstacle to miniaturization of the controller.
Also, when a relatively large-size relay is installed on a substrate etc., its center of gravity is separate from the installation surface (becomes high) and the relay is susceptible to various vibrations (engine vibration or travel vibration) of a vehicle in which the electric motor-driven power steering apparatus is mounted, and a connection place of the relay may break and also a contact of the relay may weld and in case that the contact welds, prevention of an increase in the necessary steering force due to an ON fault of the FET or protection against connection by wrong polarity of the battery became impossible and it resulted in an obstacle to an aspect of an improvement in reliability of the controller.